The present invention relates to ultrasonic medical imaging and in particular to an apparatus and method for making ultrasonic elastography measurements.
Elastography is a new imaging modality that reveals the stiffness properties of tissue, for example, axial strain, lateral strain, Poisson's Ratio, Young's Modulus, and other common strain and strain-related measurements. The strain measurements may be collected over an area and compiled as a two-dimensional array of data, which may then be mapped to a gray or color scale to form a strain “image”. Analogously, strain measurements may be collected over a volume displayed either three-dimensionally or as a series of stacked two-dimensional images.
In quasi-static elastography, two images of the tissue may be obtained by the ultrasound device in two different states of compression, for example, no compression and a given positive compression. The tissue may be compressed by an external agency such as a probe or the like, or may be compressed by muscular action or the movement of adjacent organs. Strain may be deduced from these two images by computing gradients of the relative local shifts or displacements in the images along the compression axis. Quasi-static elastography is analogous to a physician's palpation of tissue in which the physician determines stiffness by pressing the tissue and detecting the amount the tissue yields under this pressure.
In dynamic elastography, a low-frequency vibration is applied to the tissue and the tissue vibrations are measured, for example, using Doppler detection.
Typically quasi-static elastography produces a strain measurement only along the axis of compression. However, lateral strain or elevational strain (both perpendicular to the axial strain) may be of value both in deducing qualities like Poisson's Ratio and in countering the effects of lateral motion in de-correlating the axial displacement of the tissue. In addition, shear strain images can also be obtained.
A number of different methods have been used to obtain lateral strain in quasi-static elastography. Some of these techniques assume knowledge about the compressibility of the tissue (for example, Poisson's ratio), and thus cannot be used for measurements in which such tissue properties are to be determined. U.S. Pat. No. 6,270,459 to Ophir et al describes a technique which interpolates between successive axial rays or echo signals to provide a basis for horizontal displacement measurement using a correlation technique.
U.S. patent application Ser. No. 10/765,293 by the present inventors, entitled: “Ultrasonic Elastography With Angular Compounding” and hereby incorporated by reference, provides a method of obtaining both axial and lateral strain using multiple angles of ultrasonic measurement and an angle-dependent weighting factor based on an assumed value of the Poisson's ratio The compounding of the measurements from multiple angles improves the accuracy of the strain determinations.